The causal agent of sudden oak death
(SOD, also know as Phytophthora canker disease), Phytophthora ramorum, was first
identified in 1993 in Germany and The Netherlands on ornamental rhododendrons. P.
ramorum was isolated in June 2000 from dying trees in California. Since its discovery
in North America, P. ramorum has been confirmed in forests in California and Oregon
and in nurseries in California, Oregon, Washington, and British Columbia.

Origin

The geographic origin of P.
ramorum is unknown. Before the mid-1990s, there were no reports of this species in the
United
States or Europe. P. ramorums limited known geographical distribution in
relation to its hosts distribution suggests it was recently introduced into the
United States. The European and North American populations are thought to be distinct
populations transported independently from another location, perhaps the site of origin.

Host Range

P. ramorum causes two types of
diseases, bark cankers that may kill the host and foliar blights that may serve as a
reservoir for the pathogen. In response to the identification of P. ramorum, the
United States Department of Agriculture Animal and Plant Health Inspection Service
(USDAAPHIS) has developed a list of regulated hosts. Visit http://www.ncpmc.org/sod
for the most current and complete lists of regulated and associated hosts.

The spread of SOD likely occurs
through infected plant material, rainwater, and soil. Foliar hosts may play an important
role in the transmission of P. ramorum to bark canker hosts. Data suggest foliar
hosts support spore production and foliar infection may precede bark canker host
infection. Moist, cool, windy conditions are thought to spread the pathogen by dispersing
spores from the leaves of foliar hosts. Transport of infected foliar host plants may aid
the spread of SOD throughout the United States.

Symptoms and Identification

Bark canker hosts infected with P.
ramorum have large cankers on the trunk or main stem accompanied by browning of
leaves. Tree death may occur within several months to several years after initial
infection. Infected trees may be infested with ambrosia beetles (Monarthrum scutellare and
M. dentiger), bark beetles (Pseudopityophthorus pubipennis), and sapwood
rotting fungus (Hypoxylon thouarsianum). These organisms may contribute to the
death of the tree. Infection on foliar hosts is indicated by dark grey-to-brown lesions
with indistinct edges. These lesions can occur anywhere on the leaf blade, in vascular
tissue, or on the petiole. Petiole lesions are often accompanied by stem lesions. Some
hosts with leaf lesions defoliate and eventually show twig dieback.

Monitoring and Management
Recommendations

Symptoms on affected hosts vary
considerably by species and it is difficult to differentiate P. ramorum-infected
plants from those infected by other pathogens. If you suspect trees or plants are infected
with P. ramorum, contact your states university diagnostic laboratory or
Department of Agriculture diagnostic laboratory immediately (contact university extension
personnel for the address of the diagnostic laboratory). Plants infected with P.
ramorum should be destroyed because no chemical control measures are currently
available. Because P. ramorum is a regulated organism, destruction and disposal
protocols will be coordinated by state regulatory officials. If diagnosticians confirm P.
ramorum infestation of plants at nurseries or other commercial landscape facilities,
an Emergency Action Order from APHIS may be issued. The order may require that P.
ramorum-infected plants and all susceptible plants within 2 meters of infected plants
be destroyed and that all susceptible plants within 10 meters be held for 90 days until
inspected.

For more information on sudden
oak death, see http://www.ncpmc.org/sod.